The present invention relates to a chromate-free etching process and composition for the treatment of aluminum parts to produce surfaces, which can be adhesively bonded to other parts, e.g. by means of an epoxy type adhesive, to produce durable, long-life bonds.
In the past the usual procedure for preparing aluminum parts for adhesive bonding involved etching the aluminum parts, prior to application of the adhesive bonding agent, with a solution of sulfuric acid and sodium dichromate in water. The use of such etching solutions, which contained considerable concentrations of chromate, resulted in contamination of the air in the plant as well as the rinse waters used in processing the aluminum. Although the removal of such chromate contamination is possible, it is expensive and not always completely effective, which results in toxic hazards to workers and the release of toxic materials into surrounding watersheds. At present the spent etchant and chromate containing rinse waters are treated with sodium bisulfite to reduce the chromium to the trivalent state and then neutralized to pH7 to precipitate the trivalent chromium compound, after which the treated waste liquor is pumped into a settling pond where the precipitated chromium compound settles out as sludge. The precess is expensive and often ineffective if accidental spills occur. Blowers and ducts are often used to sweep away the vapors over the processing tanks, but they are not totally efficient so that the workers are exposed to the fumes which are toxic and carcinogenic.